Maxhouse97

Completed the rebuild, still have the problem

Maxhouse97

Anyone know of a system/sensors that a race shop might have to measure clamping force at each caliper upon brake application? My only other thought for troubleshooting is to somehow do that to see if one caliper is not putting out the force of the other. Or to see if there is a line blockage somewhere.

When I installed my 996 brakes, my shop essentially "cleaned out" the stock 87 Carrera brake bias switch that limited brake pressure to the rears (because the 996 setup did not need this). I thought perhaps they somehow messed that up, but that should only effect fore and aft, not left and right.

I'm quite annoyed - appreciate any help you smart folks could give me. May just turn it over to Wright Motorsports (a good race shop) in my area. Have a call into them now. At least my calipers are freshly rebuilt.

car_slave

What pads and tires are you running? Is it possible that you have too much front brake for that car?

ProCoach

Wright's a great shop. Here's a diagnostics article:http://www.brakeandfrontend.com/adva...gnostic-tools/

The tools exist to stick in the caliper and measure clamping force, both front to rear and side to side (usually done in pairs).

Also, don't be so sure that the proportioning valve is JUST front to back. Many brake systems from that era function diagonally for split circuit failure protection.

NYC993

I'm not familiar with 996 callipers, are the pistons the same? My 993tt has different size pistons to keep even wear on the pad, so flipping them would not work.

jj1

Sorry if this is obvious but I didn't see it in your trouble shooting post, have you switched the front wheels side to side? Maybe the new calipers shifted the bias forward substantially and its exacerbating the grip difference between the front tires.

Maxhouse97

Again the lock-up issue is between left and right, so pads between the back and forth is not a factor in my view.

I've tried different wheels and tires, so that is not the issue.

Prococah - thanks. I'll check out that thread. I really think the proportioning valve may be the problem, as again my mechanic said he would essentially remove the internals that limited brake pressure to the rear and leave the body intact. If there was a second circuit or if he didn't evenly clear them that could be the problem. I am also encouraged by the fact that a tool exists to measure brake force. More to follow.

flatsics

Check the corner balance... Also, if you have adjustable sway bars you may have too much pre-load on one side causing lock up.

flatsics

Porsche uses the proportioning valve on the rear circuit only.
It is very simple to remove the guts of the proportioning valve and reinstall it.

flatsics

Good proportioning valve internals thread.

https://rennlist.com/forums/964-foru...look-like.html

flatsics

Sorry, didn't read enough... So you're corner balanced.
I had this problem before with a front sway bar with adjustable end links. Make sure there is no pre-load on a flat surface.

Maxhouse97

So I had the car corner balanced before, and eliminated that as a cause. But now that I look back at the numbers, could this be it?

With 185 lb driver weight:

LF 625
RF 554
LR 922
RR 849

That's 70 lbs less on the front right than the front left. Could that be the cause? I think the tech said they couldn't do much more to balance it. What are my options? Or is this amount of left to right variation normal? I've had a person in the right seat and still have the problem, which is another reason why I ruled this out...

car_slave

You need to put those numbers into a cross weight calculator to see the real picture.

https://robrobinette.com/corner_weight_calc.htm


You're at 50% cross.
40%-60% front to rear
52%-48% left to right

Maxhouse97

BTW this is the tool I was looking for:

http://www.ntxtools.com/network-tool...FdRZhgodlcwLlA

Glad to see it exists! Now I need to find a shop that has one, so I can rule out the brake system.

Maxhouse97

Ok so then that probably isn't the problem?